LEDs are semiconductor elements for converting electric energy to light such as ultraviolet light or visible light which is then radiated. LED lamps comprising the LED chip sealed, for example, with a transparent resin are used in various fields. LED chips are semiconductor elements and thus have a prolonged service life, a high level of reliability, and, when used as a light source, can reduce replacement work and the like. Accordingly, LED chips have become extensively used as components constituting various display devices, for example, for portable communication equipment, PC peripheral devices, OA equipment, domestic electric appliances, signal devices, various switches, and backlight-type display boards.
The color tone of light emitted from the LED lamp is not limited to the luminescence wavelength of the LED chip. For example, light in a visible light region from blue light to red light depending upon applications, where the LED lamp is used, can be provided by coating a phosphor onto the surface of the LED chip, or by incorporating a phosphor in a transparent resin for sealing the LED chip. In particular, the white LED lamp has rapidly been used in applications such as backlight in a display part of portable communication equipment and on-vehicle lamps, and the applications are expected to be significantly expanded as an alternative to fluorescent lamps in the future.
At the present time, an LED lamp comprising a combination of a blue light emitting LED with a yellow light emitting phosphor (for example, YAG) and an LED lamp comprising a combination of an ultraviolet light emitting LED with a mixture of a blue light emitting phosphor, a green light emitting phosphor, and a red light emitting phosphor are known as white LED lamps which are used widely or experimentally used. At the present time, as compared with the latter white LED lamp, the former white LED lamp using a blue light emitting LED has better brightness properties and the like and thus is used more widely. The former white LED lamp has a drawback that the emitted light is viewed as a yellowish light in some viewing direction and, when the emitted light is projected on a white face, uneven yellow or blue color appears. For this reason, the former white LED lamp is sometimes called as “pseudo white.” Regarding the average color rendering index representing the quality of white light as well, the former white LED lamp is up to 70 to 75.
On the other hand, the latter white LED lamp using an ultraviolet light emitting LED is inferior in brightness to the former white LED lamp but causes no significant uneven luminescence and projected light. Accordingly, the latter white LED lamp is expected to become a main stream of white lamps for lighting applications in the future, and the development of the latter white LED lamp has been rapidly forwarded. In the white LED lamp using an ultraviolet light emitting LED, in addition to the properties of each color light emitting phosphor, a combination of these phosphors affects lamp characteristics such as color rendering properties and brightness. Accordingly, studies on the selection of each of blue light, green light, and red light emitting phosphors and combinations thereof have been forwarded.
For example, nonpatent document 1 describes a white LED lamp comprising a combination of an ultraviolet light emitting LED, with a europium(Eu)-activated halophosphate phosphor or a europeum(Eu)-activated aluminate phosphor as a blue light emitting phosphor, a copper(Cu)-and aluminum(Al)-activated zinc sulfide phosphor or a europium(Eu)-and manganese(Mn)-activated aluminate phosphor as a green light emitting phosphor, and a europeum(Eu)-activated yttrium oxysulfide phosphor as a red light emitting phosphor. Further, patent document 1 describes that a europium(Eu)-activated halophosphate phosphor or a europium(Eu)-activated aluminate phosphor, a europium(Eu)-and manganese(Mn)-activated aluminate phosphor, and a europium(Eu)-activated lanthanum oxysulfide phosphor are used as a blue light emitting phosphor, a green light emitting phosphor, and a red color emitting phosphor, respectively.
[Non-patent document 1] MITSUBISHI CABLE INDUSTRIES, LTD., Newsletter Vol. 99, July, 2002.
[Patent document 1] Japanese Patent Laid-Open No. 073052/2000